System for suppressing interfering currents



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PULSATING LOAD DIRECT cumzm R Feb. 10, 1931. P. H. CRAIG l.792,001

SYSTEM FOR SUPPRESSING INTERFERING CURRENTS riginal Filed. 9, 1929 A LO/ID zero Luve gwuemtoc Palmer H Craig (1R0: nu

Patented Feb. 10, 1931 UNITED STATES-PATENT OFFICE rarimmnurrr owe, or cmcmlta'rr, onro SYSTEM FOR SUPPBESSING INTERFERING CUBBENTS Application filed February 9, 1929, Sbrial No. 888,828. Renewed April 241:, 1930.

My invention relates to circuit arrangements for suppressing the ripple or alternating current component in a pulsating direct current supply source.

My invention also involves a novel method of suppressing the ripple in a source of pulsating direct current.

My invention is particularly useful in suppressing commutator ripples in circuits supplied with direct current from a commutator generator.- It may also be used to suppress the ripple in the output circuit of an alternating current rectifier system.

An object of my invention is to devise a circuit network which when interposed between a' source of pulsating direct current audits load will substantially eliminate the "ripple current in the load circuit.

A further object of my invention is to de-v vise a circuit network interposed between a source of pulsating direct current and its load by which current is supplied to the load over a plurality of paths, and the ripple current in one path is retarded (or advanced) in phase by 180 degrees with respect to the ripple current in another path.

My invention is illustratedlin the accompanying drawing in which Figure 1 shows the invention applied to a single source of pulsating direct current; Figure 2 shows the invention applied to a direct current generator having'two commutators; I 1 g Figure 3 is a diagram of a filter employed in the explanation of the operation of my I invention Figures 4 and. 5 show different formsof filters which-may be employed in my invention' Figure 6 shows the relation between the currents in the two supply paths and the load current where the source of supply is a commutator generator; 7

'Figure 7 shows the relation between the various currents where the source of supply is an alternating current rectifier,

aload which is to be supplied with steady difgi'ven by thefollowing equation:

rect current from source A. The source A is connected to the load B by two parallel paths, one of which includes filter sections F1 and F2 connected in cascade, and the other path-includes a variable resistance R. Filter sections F1 and F2 comprise a filter of the laddertype, that is, a filter made up of recurring sections of series and shunt impedances. Filter networks of this type possess the property of producing a phase displacement in alternating currents .transmitted by the filter, and the phase displacement is proportional to the number of sections embodied in the filter.

The phase shift produced by a ladder filter may be determined in accordance with the following mathematical formulae:

Referring. to Figure 3, there is shown a general circuit diagram for a filter of the ladder type in which the series impedances are indicated by the reference characters Z1 and characters Z2. The portion of the filter included in the dotted square is known as a filter section. In a uniform filter network of this kind, the propagation constant 7 is cosh 1 1 Where Z1 and Z2 are pure reactances,

7 a Where (s -attenuation constant per section dians) Within a pass band, a 0 and B I 0 Within a stop band, 0:10 and 3=0 Y Hence for phase shift purposes, the ire: lliency in use lies within a pass'band of the ter. For a low-pass filter shown in Figure 4,-

the shunt impedances are represented by the Bf-phase shift per section (ra- Since cosh y is real, the last term vanishes and, cosh =cosh 0: cos ,8.

For the pass band, a=0, and cosh=1.

Therefore cosh y=COS ,B=1%w LG. I

'm=21rf, where f is the frequency of the ripple.

By proper choice of the product LC, B, or

where is the equivalent load resistance.

The pulsating current delivered by source A maybe considered ascomprising a steady direct current and an alternating current, or-

ripple, superimposed upon the steady current. In. accordance with the foregoing design principles, I include sufiicient sections in the filter to produce a total phase displacement of 180 in the ripple current transmitted over the upper path to the load. Since the lower path includes simply a pure resistance R, the phaseof the ripple current transmitted over this path remains unchanged. Accordingly, if resistance R is adjusted until the amplitude of the ripple current supplied over the lower path is the same as that over the upper path, the two currents will combine at the load terminals in opposite phase relation and will neutralize each other.

The filter inserted in the upper conducting path may be formed in two section F1 and F2 as shown, each producing one-half the total phase shift, or a single filter section of proper design may be employed to produce the desired phase shift. It will be understood, of course, that more than two filter sections may be employed, each producing a proportional fraction of the total desired phase shift.

I prefer to use low-pass filters of the type shown in Figure 4, but it will be understood that high-pass filters of the type shown in Figure 5 may be employed if desired. In case filters of the type shown in Figure 5 are used, only the ripple current will be transmitted over the path including the filters, and both direct and ripple current will be transmitted over the lower path.

The source A. may be any source of pulsating current, such as a commutator type generator, or a source of rectified alternating current.

In Figure2 I have shown my invention applied to a commutator type of generator provided with a commutator at each end of the. armature. In this case the ripples produced by the two commutators are of the same frequency and are synchronous. The operation of the arrangement .shown in. Figure 2 be apparent from the description of the op eration of Figure 1.

In Figure 6, I have shown the current relations existing-in the circuit of Figure 1. The solid line IR indicates the current transmitted over the lower path through resistance R, and the dotted line IF indicates the current transmitted through the filter path.

It will be seenthat the two ripple currents superposed upon these two supply currents are opposite in phase and substantially neutralize each other. The resultant load current is shown by the line Ida, which is the sum of lines IR and IF, and it will be seen that the resultant ripple in the load circuit is of higher frequency and greatly reduced amplitude.

Figure 7 shows the current relations existing in the circuit when the source of supply is a full-wave alternating current rectifier.

It will be apparent to one skilled in the art that the filter inserted in the upper path connecting the source A with the load B is equivalent to a long transmission line having an effective length greater than the lower path by one-half wave length of the ripple current. It will also be understood that filters may be inserted in both paths, but one path must contain a greater number of filter sections in order to produce a relative phase shift of 180 degrees.

The ripple current may not always be a simple harmonic current, but may be a complex wave made up of a fundamental and several harmonics. It will be understood that where I refer to a phase shift of 180 or to a half-wave length of the ripple current, this has reference to the most predominant component of the ripple. Where the rippleis very complex, and this is probably the usual case, the phase-shift required may not be exactly 180, but willbe of such magnitude as to reduce the interference to a minimum.

The physical embodiment of my invention may vary in many details without departing from the spirit of the invention.

What I claim is: a i v 1. In combination, a source of pulsating direct current, a load circuit, a circuit connecting said source and load, a second circuit connecting said source and load, a filter network in one of said circuits for causing a relative phase shift of 180 degrees between the ripple currents in the two connecting circuits. I

2. In combination, a source of pulsating direct current, a load circuit, two parallel circuits connecting said load to said source, one of said connecting circuits having an effective length greater than the other connecting circuit byone-half wavelength of thev ripple current from said source.

3. In combination, a source 'ofdi'rect current containing a ripple current, a load, two

direct current supply circuits connecting said load to said source, means for producing a relative phase shift of 180 degrees between the ripple currents transmitted over the two connecting circuits, and means to equalize the amplitude of the two ripple currents.

4. The method of suppressing the ripple current in a load circuit supplied from a source of pulsating direct current which consists in, supplying current to the load over two parallel paths one of which has an effective length greater than the other by onehalf wave length of the ripple current, and

' equalizing the amplitudes of the two ripple currents transmitted.

5. The method of suppressing the ripple current in a load circuit supplied from a source of pulsating direct current which consists in, supplying the pulsating direct current to the load over two parallelipaths, and

causing a relative phase shift of 180 degrees between the ripple currents in the two paths.

6. In combination, a source of direct current containing a ripple current, a load, two direct current supply circuits connecting said load to said source, means for producing a relative phase shift of 180 degrees between the ripple currents transmitted over the two connecting circuits.

7. In combination, a source of pulsating direct current, a load circuit, two parallel circuits connecting said load to said source, and a filter network comprising recurring sections of series and shunt impedances connected in one of said circuits for causing a relative phase shift of 180 degrees between the ripple currents in the two connecting circuits.

8. In combination, a source of direct current containing a ripple current, a load, a direct current supply circuit connecting said load to said source, and a second circuit connected between said load and said source independently of said supply circuit for supplying a ripple current to said load to substantially balance the ripple current supplied by said direct current sup 1y circuit.

In witness whereof I a m si ature.

PALMER H T RAIG. 

